In one known structure for reinforcing an outer panel of a railway vehicle, vertically and horizontally arranged frame members are placed on and weld-joined to the interior surface of the outer panel to reinforce the outer panel (see, for example, Patent Document 1). In another exemplary structure, vertical and horizontal frame members b and c are disposed on and weld-joined to an outer panel a, as shown in FIGS. 6A and 6B. In these structures, spot welding shown in FIG. 7A is used for weld-joining. However, since a load of 1 t is applied during the spot welding process, indentations are formed on the exterior surface, resulting in the deterioration of the appearance. In addition, much time and labor are required for joining, and the cost increases. Since a large load is applied, a large apparatus is required, and an expensive copper-made electrode plate d must be used, so that the cost for the equipment is high.
In view of the above, in one known method, laser welding shown in FIG. 7B is used for weld-joining (see, for example, Patent Document 2).    [Patent Document 1] Japanese Patent Application Laid-Open No. Hei. 9-30414.    [Patent Document 2] Japanese Patent No. 2843899.